Magnetron sputtering device

ABSTRACT

A target arrangement for use within a vacuum sputtering zone comprising: a primary target ( 1 ) comprising material to be sputtered and an auxiliary target ( 2 ) of ferromagnetic material, the targets being located relative to one another such that upon application of a magnetic field across the surface of the targets, the magnetic field over the primary target is confined into an area of high homogeneous magnetic field strength substantially parallel to the surface of the primary target, to achieve uniform erosion across said area of target during sputtering. Preferably the primary target and auxiliary target are spaced apart by a gap ( 12 ), to enhance the confinement of magnetic field. The auxiliary target may have a surface which is raised beyond a surface of the primary target. The primary target may be of ferromagnetic or non-magnetic material.

The present invention relates to magnetron sputtering, more particularlyto targets for magnetron sputtering, to a device including such a targetand to a method of magnetron sputtering. Particularly, in certainembodiments the invention relates to magnetron sputter sources which areable to sputter off from ferromagnetic targets using larger thicknessthan conventional devices.

Magnetron sputtering of ferromagnetic materials such as metallic cobalt,nickel and iron is limited due to the high magnetic permeability ofthese metals. In the case of iron, for instance, a maximum of 1.5 mmtarget thickness can be sputtered using conventional methods. A targethaving a thickness of greater than 1.5 mm gives rise to a magnetic fieldof insufficient strength to produce the required plasma for sputtering.

A further drawback of conventional sputtering methods is that about 90%of the target material is trapped by the magnetic field and cannot beetched. These problems result in a process which is costly andinefficient in terms of target utilisation and wastage of time. Sinceonly thin targets can be used, they must be replaced frequently,resulting in the inconvenience of stopping the process at regularintervals and then regenerating a vacuum after replacement. This processis neither cost effective nor time efficient.

Furthermore, existing methods cannot easily control the escape of ionsfrom the target area towards the sample which is being coated, thereforefilm properties may vary from run to run. This poses limitations on thetechnology for which the film can be used, particularly for example formagnetic recording media.

It is an object of the present invention to provide a device and methodfor sputtering which addresses one or more of the above mentionedproblems. Embodiments of the present invention may be based on magnetronsputtering sources where the ferromagnetic target to be sputtered iscomposed of at least two parts. Generally a gap between the severalferromagnetic parts can be present. By selecting the relative positionand polarity of several magnetic means in relation to the ferromagnetictarget parts it is possible to close magnetic field lines above the mainferromagnetic target material. Generally at least one of theferromagnetic target parts can be above the surface level of the otherpart so a vault is created in which the electric and magnetic fieldconfinement are enhanced. The magnetic field in the vault would have ingeneral two main areas, one of high magnetic field strength and anotherone of a lower very homogeneous magnetic field strength. The magneticand electric field design allows the sputtering at very low pressures.The sputtering of ferromagnetic material is stable during sputtering andlow defects are created on the sputtered film.

According to the invention in a first aspect there is provided a targetarrangement for use within a vacuum sputtering zone comprising:

-   -   a primary target comprising material to be sputtered and    -   an auxiliary target of ferromagnetic material, the targets being        located relative to one another such that upon application of a        magnetic field across the surface of the targets, the magnetic        field over the primary target is confined into an area of high        homogeneous magnetic field strength substantially parallel to        the surface of the primary target, to achieve uniform erosion        across said area of target during sputtering.

The area of high homogeneous magnetic field need not prevail across theentire surface of the primary target, but will generally cover a partialarea of the surface of the primary target.

Preferably, the primary target and auxiliary target are spaced apart bya gap, to enhance the confinement of magnetic/electric field andsputtering operation.

Preferably, the auxiliary target has a surface which is raised beyond asurface of the primary target. This creates a vault in which theconfinement of magnetic field is enhanced. Preferably, the raisedsurface of the auxiliary target extends beyond the surface of theprimary target by a height which is less than or equal to the width ofthe primary target. However, the auxiliary target may alternatively bearranged to have an exposed surface which is level with that of theprimary target, for example when the primary target is of a non-magneticmaterial. In cases where there is more than one auxiliary target, eachmay be raised to a different height relative to the primary target.

Preferably, in each of the above mentioned aspects of the invention,there are a plurality of auxiliary targets. There may also be more thanone primary target. In such case, preferably the auxiliary targets arelocated adjacent and between each of the primary targets.

Preferably the primary and auxiliary targets are of the same material.This avoids the risk of contamination resulting from any erosion of theauxiliary target during the sputtering process. However, it is alsopossible for the primary and auxiliary targets to be of differentmaterials. Unlike the auxiliary target, the primary target need not beof magnetic material.

The targets may be planar or non-planar. Additionally, they may becircular, rectangular or any other geometry when viewed as a plan. Theprimary target may be profiled, e.g. to have a slotted surface, in orderto decrease the operative pressure at which sputtering can take place.

The preferred dimensions of the targets and their relative positionswill depend upon the materials used and the sputtering conditions. Thegap between the primary target and auxiliary target is preferably notmore than 10 mm wide, more preferably not more than 5 mm wide. Yet morepreferably, the gap is between 1 and 2 mm in width.

The gap need not divide the primary and auxiliary targets entirely, butmay extend only partially between the targets, along their surface. Thepreferred depth of the gap varies depending upon materials andconditions. Preferably, the gap has a depth of at least 30% whencompared with the thickness of the primary target. More preferably, thegap extends down to at least 70% of the thickness of the primary target.Most preferably, the depth of the gap is between 90% and 100% of thethickness of the primary target. Where the gap does not extend to thefull thickness of the primary target, the primary and auxiliary targetscan be integral and thus of the same material. Alternatively, theprimary and auxiliary targets are discrete targets which can be removedand replaced independently. This is particularly advantageous since theprimary target will be eroded faster than the auxiliary target.Generally, over a period of time in which the primary target is replaced10 times, the auxiliary target will only need to be replaced once.

Preferably the primary and auxiliary targets are mounted on a backingplate. The backing plate may be planar, so that the targets are mountedat the same level, or may for example be stepped so that the primary andauxiliary targets are mounted at different base levels. In someembodiments having a plurality of auxiliary targets, it is preferablethat these be mounted on the backing plate from different base levels.There may be more than one backing plate, and the primary and auxiliarytargets need not be mounted on the same backing plate.

According to a second aspect of the invention there is provided a devicefor magnetron sputtering comprising the target arrangement of the firstaspect of the invention, and further comprising:

-   -   magnetic means for creating a magnetic field in a region above        the targets,    -   the magnetic field over the primary target being confined into        an area of high homogeneous magnetic field strength        substantially parallel to the surface of the primary target.

Preferably, the magnetic means are provided directly behind, or in theproximity of the auxiliary target to produce the required magnetic fieldstrength distribution.

According to a third aspect of the invention there is provided the useof the device of the first aspect for magnetron sputtering.

By confining a high magnetic field strength to a specific desired area,the present invention enables the use of thicker targets when comparedwith conventional methods. Generally, the invention allows the use of atarget thickness of 5-10 times that of conventional sputtering. Forexample, a primary target having thickness of 5-6 mm may be used in thecase of iron, while a nickel target of up to 10 mm thickness may beemployed. As a result, the target need not be replaced as frequently,offering savings in time and expense.

Additionally, owing to the shape of the magnetic field generated, thepresent invention enables a much higher target utilisation, which can begreater than 50%, compared with conventional methods which generallyonly allow less than 10% of the target to be eroded before replacement.This further reduces wastage of materials, thus improving cost and timeefficiency.

A further advantage of the present invention is that it allowssputtering at very low pressures, which enables the production of betterquality coatings having a lower density of defects.

The invention will now be described in detail, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a cross section of an arrangement according to a firstpreferred embodiment of the invention.

FIG. 2 is a cross section of an arrangement according to a secondpreferred embodiment of the invention.

FIG. 3 is a cross section of an arrangement according to a thirdpreferred embodiment of the invention.

FIG. 4 a is a cross section of an arrangement according to a fourthpreferred embodiment of the invention.

FIG. 4 b is a detailed cross section of the embodiment of FIG. 4 a.

FIG. 4 c is a plot of flux density against distance with reference tolocations along the arrow line of FIG. 4 b.

FIG. 5 is a cross section of an arrangement according to a fifthpreferred embodiment of the invention.

FIG. 6 is a cross section of a comparative arrangement, shown forcomparison with the embodiments of the present invention.

FIG. 7 is a cross section of an arrangement according to a sixthpreferred embodiment of the invention.

FIG. 8 a is a cross section of an arrangement according to a seventhpreferred embodiment of the invention.

FIG. 8 b is an enlarged detail of the embodiment of FIG. 8 a, showingthe distribution of magnetic field.

FIG. 9 a is a cross section of an arrangement according to an eighthpreferred embodiment of the invention.

FIG. 9 b is an enlarged detail of the embodiment of FIG. 9 a.

FIG. 10 is a cross section of an arrangement according to a ninthpreferred embodiment of the invention.

Referring to FIG. 1, this shows a magnetic array and a ferromagnetictarget, the target having three parts. The primary target 1 provides themain material to be sputtered. Separating and surrounding the primarytarget 1 are two other target parts, an outer auxiliary target 2 and aninner auxiliary target 3. A gap 12 separates the primary target 1 fromeach of the auxiliary targets 2 and 3.

Below each of the auxiliary targets 2 and 3 are located magnetic means4, comprising magnetic poles 4 a and 4 b. A casing 5 separates the lowpressure zone, in which the targets are located, from atmosphericconditions. In the embodiment shown, the casing S includes an integralbase plate onto which the targets 1, 2 and 3 are mounted.

The auxiliary targets 2, 3 are raised above the plane of the primarytarget 1. The relative position of magnetic poles 4 a and 4 b withrespect to the three different target parts enables the magnetic fieldto be conducted across in field lines 6, to produce a closed S fieldbetween the auxiliary targets 2 and 3. As a result, a stable,homogeneous, low magnetic field is produced above the surface of thecentral region of the primary target, causing this area to be eroded byplasma while the outer edge parts of the primary target experiencenegligible erosion. Similarly, the auxiliary targets 2 and 3 undergoonly a small amount of erosion. The gradual erosion of the primarytarget occurs in a uniform manner so that some 50% of the targetmaterial may be used before replacement. In contrast, the auxiliarytargets 2, 3, which are not substantially eroded, need not be replacedfrequently, if at all. In summary, the electric field created when thetarget is biased during the sputtering process, together with themagnetic field structure, enables sputtering at very low pressures.

FIGS. 2 and 3 show the same arrangement of targets and magnetic means,the profile of the auxiliary targets 2 and 3 having been modified toproduce a different electric and magnetic field distribution. Bymodifying the profile of the auxiliary targets, the extent of theirerosion by sputtering can be minimised. This is particularly desirablewhen the auxiliary targets are of different material from that of theprimary targets, since erosion of the auxiliary targets would lead tocontamination of the coating material. With a square profile, such as inFIG. 7, or a protruding profile such as is shown in FIG. 1, the uppercorner area of the auxiliary target tends gradually to become eroded,with some deposition of material occurring in the region below thisarea. This unwanted erosion can be reduced by profiling the upper cornerof the auxiliary target along an incline as shown for example in FIGS. 3and 4 a. FIG. 2 shows a further possible profile which may be employed,though the shape depicted would again result in some unwanted erosion.The design of the targets in FIG. 3 also allows easier replacement ofthe primary targets.

In the embodiment shown in FIG. 4 a, additional magnetic means 7 arepresent to enhance saturation of magnetic field and to boost magneticfield strength, as shown by magnetic field lines 6. The saturation ofthe primary target 1 is thus enhanced, enhancing plasma confinement bymagnetic field lines 6. This enables a yet thicker primary target to beused for sputtering.

Referring to FIG. 4 b, the magnetic field lines are shown in moredetail. Magnetic field strength (Bx) wag measured in the region abovethe primary target 1 along the arrow line 8 a-8 b-8 c, and the resultsplotted in FIG. 4 c. Referring to FIG. 4 c, the points in the regionslabelled 8 a′, 8 b′ and 8 c′ correspond respectively to the regionsalong sections 8 a, 8 b and 8 c of FIG. 4 b. As shown, the magneticfield is trapped in areas of high strength at 8 a and 8 c and areas oflower, homogeneous field strength at 8 b.

The embodiment of FIG. 5 resembles that of FIG. 4 a except that thecentral auxiliary target 3 is not present, the target having only twoparts. As shown by the magnetic field lines 6, the arrangement stillproduces a closed magnetic field in two regions across the primarytarget 1, though the magnetic confinement has been reduced when comparedwith FIG. 4 a.

In FIG. 6 however, the gap between the primary and auxiliary targets hasbeen closed so that a single target 1 is now present. The absence of agap results in a much lower magnetic field strength in the region abovethe target, as shown by lines 6. Consequently the special magneticconfinement shown in the other figures has been lost, and the desiredresult of the present invention is not achieved.

In contrast, FIG. 7 shows a further possible variation within the scopeof the invention. The profile of the primary target 1 has again beenmodified to alter the exact shape and strength of the magnetic field.

FIGS. 8 a and 8 b illustrate a possible variation in the surface profileof the primary target. The surface has a slotted profile, which mayfurther reduce the pressure at which sputtering can be performed.

In FIGS. 9 a and 9 b, the gaps 12 between the primary and auxiliarytargets do not divide the targets entirely. Adjacent the point ofcontact with the base plate, the targets contact each other. The primarytargets are provided with extending flanges 16 which contact theauxiliary target. This arrangement has the advantage that the targetscan be easily and correctly located on the base plate during assembly.Also the primary target can be removed and replaced separately from theauxiliary targets when necessary.

In FIG. 10, the primary target 1 is of a non-magnetic material. In thisembodiment, a gap between the primary and auxiliary targets is notnecessary, and the targets are shown to but against each other. An anode20 is located over the outer auxiliary target 2. This blocks plasma fromcontacting the auxiliary target 2 and stops erosion of its surface, thusacting as a shield. This has the effect of concentrating the plasma overthe primary target 1 instead, where it is desired for etching. The anodemay be of any suitable material, for example titanium, aluminium orstainless steel. The anode can be grounded to earth, floating or can bebiased to have higher potential than the cathode.

A small amount of redeposition of material occurs on the side surfaces 3a of the central auxiliary target 3, and this prevents unwanted erosionat the surfaces of the auxiliary target.

Also shown in FIG. 10 is an additional ferromagnet 22, which has theeffect of shifting the magnetic field lines 6 to have a flatter profile,more generally parallel to the surface of the primary target 1. Thisalters the distribution of erosion across the surface of the primarytarget, producing a flatter, more even profile of erosion which enablesa larger percentage yield of sputtered material and more efficienttarget utilisation.

The above description merely sets out various preferred embodiments ofthe invention, and it will be appreciated that further modifications canbe applied without departing from the scope of the invention as claimed.

1. A target arrangement for use within a vacuum sputtering zonecomprising: a primary target comprising material to be sputtered and anauxiliary target of ferromagnetic material, the targets being locatedrelative to one another such that upon application of a magnetic fieldacross the surface of the targets, the magnetic field over the primarytarget is confined into an area of high homogeneous magnetic fieldstrength substantially parallel to the surface of the primary target, toachieve uniform erosion across said area of target during sputtering. 2.A target arrangement according to claim 1 wherein the primary target andauxiliary target are spaced apart by a gap, to enhance the confinementof magnetic/electric field and sputtering operation.
 3. A targetarrangement according to claim 1 or 2 wherein the auxiliary target has asurface which is raised beyond a surface of the primary target.
 4. Atarget arrangement according to any one of the preceding claims whereinthe raised surface of the auxiliary target extends beyond the surface ofthe primary target by a height which is less than or equal to the widthof the primary target.
 5. A target arrangement according to any one ofthe preceding claims comprising a plurality of auxiliary targets.
 6. Atarget arrangement according to any one of the preceding claimscomprising a plurality of auxiliary targets which are raised todifferent heights above the primary target.
 7. A target arrangementaccording to any one of the preceding claims wherein the primary targetand auxiliary target are of the same material.
 8. A target arrangementaccording to any one of claims 1-6 wherein the primary target andauxiliary target are of different materials.
 9. A target arrangementaccording to claim 8 wherein the primary target is of non-magneticmaterial.
 10. A target arrangement according to any one of the precedingclaims wherein the targets are mounted on a backing plate.
 11. A targetarrangement according to any one of the preceding claims wherein thetargets are mounted at different base levels.
 12. A target arrangementaccording to any one of the preceding claims wherein there is more thanone backing plate and wherein the targets are not all mounted on thesame backing plate.
 13. A target arrangement according to any one of thepreceding claims wherein the primary target can be replacedindependently of the auxiliary target.
 14. A target arrangementaccording to any one of the claims 2-13 wherein the gap between theprimary target and auxiliary target is not more than 10 mm in width. 15.A target arrangement according to any one of the preceding claimswherein the gap between the primary target and auxiliary target is notmore than 5 mm in width.
 16. A target arrangement according to any oneof the preceding claims wherein the gap between the primary target andauxiliary target is between 1 and 2 mm in width.
 17. A targetarrangement according to any one of the preceding claims wherein the gapbetween the primary target and auxiliary target has a depth of at least30% of the thickness of the primary target.
 18. A target arrangementaccording to any one of the preceding claims wherein the gap between theprimary target and auxiliary target has a depth of at least 70% of thethickness of the primary target.
 19. A target arrangement according toany one of the preceding claims wherein the gap between the primarytarget and auxiliary target has a depth of between 90% and loot of thethickness of the primary target.
 20. A target arrangement according toany one of the preceding claims wherein the primary target is profiled,having a slotted surface.
 21. A device for use in a vacuum depositionprocess comprising a target arrangement according to any one of thepreceding claims.
 22. A device for magnetron sputtering comprising thetarget, arrangement of any one of the preceding claims, and furthercomprising: magnetic means for creating a magnetic field in a regionover the targets, the magnetic field over the primary target beingconfined into an area of high homogeneous magnetic field strengthsubstantially parallel to the surface of the primary target.
 23. Adevice according to claim 22 wherein the magnetic means are locateddirectly behind, or in the proximity of the auxiliary target.
 24. Use ofa device according to claim 22 or 23 for sputtering.